Porous columbium and tantalum materials



United States Patent 3,203,793 POROUS COLUMBIUM AND TANTALUM MATERIALSRobert B. Hand, Cincinnati, Ohio, assignor to E. I. du

'Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware No Drawing. Filed Jan. 28, 1963, Ser. No.254,403

4 Claims. (Cl. 75-474) This is a continuation-in-part of applicationSerial No. 137,958 filed September 14, 1961, now abandoned.

This invention relates to novel metal products which can be anodized togive improved electrodes, to processes for producing such metalproducts, and to improved electrolytic capacitors having electrodes madefrom the metal products.

A method has now been found for producing columbium and tantalum basecomponents which make possible electrolytic capacitors of highercapacitance and lower leakage currents than those previously obtainable.This method comprises heating a shaped metal body consisting essentiallyof at least one metal from the group of columbium and tantalum and atleast one alloying metal from the group of titanium and vanadium, theheating being carried out under reduced pressure, that is, belowatmospheric pressure, and at a temperature of at least 1600" C., andbelow the melting point of the shaped metal body. Under theseconditions, titanium and vanadium are volatile, having appreciable vaporpressures, and boil out or evaporate from the metal body.

As a result of this evaporation, the metal body becomes porous anddevelops an increased surface area which makes the metal more suitablefor use in electrolytic capacitors in that more effective area ofelectrode per unit Weight metal is formed. The porous surface is of sucha character as to permit the formation or deposition there on of highdielectric-strength oxide coatings during subsequent anodization.

The products of the heating process of this invention are considered tobe novel, and they are comprised of a shaped metal body having a poroussurface consisting essentially of a metal from the group consisting ofcolumbium, tantalum, and combinations thereof. They have a roughnessfactor which is at least 3 and which remains substantially unchangedupon anodization to 200 Volts.

What is important in the metal products of this invention is thecomposition of the surface exposed to anodization, rather than theoverall composition of the metal bodies. This surface is relatively purecolumbium or tantalum or a combination of these metals. In regions ofthe metal bodies away from the surface it is quite possible to haveresidual titanium or vanadium or other metals, provided such regions arenot reached and exposed .by the anodizing treatment.

The surface purity can be demonstrated by the following techniques:

The porous metal body is anodized to 200 volts at 25 C. by electrolyzingit in a 0.01% by weightsolution of orthophosphoric acid in water. Thisforms on the surface a thin layer of oxides of the metals in suchsurface.

electrolytic capacitors by wel-known methods.

This anodic film is isolated by leaching away the metal immediatelyunderlying it, using as the leaching reagent a 10% by volume solution ofbromine in methanol. The isolated film is then analyzed by emissionspectrographic methods.

By the foregoing test it is found that the anodic oxide films on metalbodies prepared-by processes of this invention have contamination levelsof not over about 10% by weight. Constituents other than columbium,tantalum and oxygen are considered to 'be contaminants in the oxidefilm. To gain the maximumirnprovement in the novel metal bodies-theconcentration of contaminants in the anodic oxide film should be belowabout 1% by weight.

The manner in which enhancement of the electrical properties of anodesprepared from novel metal products of this invention is effected may beillustrated by consideration of the novel processes as applied to Nb-Tibinary alloys. The alloys which are to be heat-treated are firstprepared by melting and remelting together weighed portions of the puremetals to obtain a homogeneous alloy button which is then rolled to afoil of uniform thickness. Electrodes cut from this foil are heatedunder vacuum so that the lower melting metal, titanium in this case,volatilizes. The vapor pressure of titanium is great enough, even attemperatures considerably below the melting point of the Nb-Ti alloy, todevelop cavities near the foil surface. The vapor pressure of titaniumin these cavities is relieved through creation of channels opening tothe outerperiphery of the specimen, and pores are formed on the surfaceextending more or less deeply into the body of the foil.

One possible explanation of the operation of the process is that thealloys become plastic at the temperatures involved. The specimen becomesincreasingly rigid as its composition changes due to the evolution ofthe more volatile metal. This increased rigidity is due to the increasein melting point of the alloy as it becomes richer in the high meltingpoint refractory metal and loses more of the lower melting, morevolatile metal. Because of this change in melting point with changingcomposition, a means exists by which vapor pores are created and frozenin place before coalescence or collapse can occur. While .thisexplanation appears to fit the facts, it is not to be considered aslimiting, the scope ofthe invention being as defined in the appendexlclaims.

Although this discussion of the process by which the porous anodematerial is formed has been illustrated by reference to a Nb-Ti binaryalloy, the same explanation is equally applicable to the formation ofporous materials wherein vanadium or a combination of vanadium andtitanium is evaporated from columbium or tantalum or from an alloycomprising a combination of these two refractory metals.

Theshaped metal electrodes are anodized for use in These methods ofanodizing are described in the literature. See, for example, Fixed andVariable Capacitors, by Dummer and Nordenberg (McGraw-Hill, New York,1960) Chap. 9.

The alloys can be prepared by blending columbium and/or tantalum withtitanium and/or vanadium in the proportion desired and melting in aninert atmosphere. A useful property of these alloys is that buttons oringots prepared in this way are homogeneous solid solutions and may beformed readily by conventional metal working operations into shapesappropriate for capacitor electrodes, such shapes including foils,sheets, wires, woven wire fabrics, or in short solid objects of anyuseful geometry in which highly developed surfaces may be achieved bythe method described above. For example, buttons of Nb-Ti or Ta-Tiprepared by direct melting may be rolled to foil directly at roomtemperature and without intermediate heat-treatment or annealing betweensuccessive rolling operations. The shapes so produced are usuallycleaned with conventional reagents before the heattreating step.

In a process of this invention, the heating period has ranged from 15minutes to 6 hours at temperatures within the range of 1600 C. to 2100C. in vacuum of about mm. Hg. The time and temperature of the heatingvary with the alloy composition, the initial thickness of the alloybody, and the degree of porosity which is desired in the alloy surface.

After heating, the porous metal body can be anodized to form a thindielectric film of oxide on its surface. The anodization is carried outat constant current density until a desired voltage is reached as in theusual practice. In the examples which follow, the dielectric oxidecoating was formed to 200 v. at a temperature of 25 C., by electrolyzingin 0.01% H PO (wt. percent).

It has been shown that formation can be effected to very high voltageswithout damage to the anodic layer. For example, a Ta-Ti derivative wasformed in 0.01% propionic acid to the scintillation voltage of theelectrolyte (550 volts) without evidence of anodic film degradation,which would be shown by increased slope in the logarithmic plot of DC.leakage vs. formation voltage. Thus, full utilization of the highvoltage characteristics of this anode material is now onlyelectrolyte-limited.

After formation, D.C. leakage through the oxide film remains at a lowlevel at room temperature. Increase in DC. leakage from 55 to +85 C. isapproximately exponential. These facts demonstrate good blockingbehavior of the anodic layer in this temperature range.

After the damage which usually occurs during application of MnOcounterelectrode in the case of solid electrolyte devices, alloyderivatives can be rehealed readily. Voltages up to 200 volts in thefirst reheal, and 120 volts in the third reheal, have been obtained onTa-Ti derived anodes.

In the case of wet electrolyte devices, the encapsulated unit can berehealed to 70% of formation voltage with preservation of very low D.C.leakage.

No significant deterioration of the anodic layer formed on a Ta-Tiderivative was observed after 6 months immersion in 35% H 80 at roomtemperature. Upon application of voltage, the initial low D.C. leakagewas rapidly reattained.

Excellent anodic characteristics have been observed on anodes preparedfrom a variety of initial compositions and under varying vacuum heatingconditions, indicating the reproducibility of film quality.

Extensive investigation has resulted in a large body of informationwhich illustrates how capacitance of an alloyderived anode varies withrespect to (1) initial alloy composition, (2) specimen geometry, and (3)temperature-time programming during vacuum heating. As a result of theseinvestigations, it is possible to choose optimum conditions for maximumcapacitance development in the case of alloy sheets. Capacitance gain orratios of capacitance to that of a plain sheet formed under identicalconditions, ranging up to 38 on thick (initially 0.025 in., finally0.015 in.) Ta-Ti derived sheet have been measured.

Capacitance gain in the alloy-derived anodes is, to a firstapproximation, independent of formation voltage up to about 500 volts.Capacitor grade Ta foil can be etched electrochemically to show gains upto 4-5 at very low formation voltages (ca. 15 volts), but the gaindisappears at formation voltages above 150 volts.

When the pertinent variables are properly controlled, capacitance perarea values can be reproduced readily in the alloy-derived anodes,indicating that final devices can be made with a high degree ofprecision, probably of the order of i5%. Capacitors made fromelectrochemically etched foil show capacitances reproducible within +20to 50%.

High quality of the anodic oxide film leads to low inherent dissipationfactors. Also, resistive losses are low because the relatively largepore size in the anodes makes possible low resistance paths in thecounterelectrode.

It appears that the high quality of the anodic layer results from a highdegree of perfection in the anode surface. High fiuidity of thespecimens during vacuum heating and Ti evolution probably results in ahighly homogeneous distribution of impurities in the lattice of theresidual metal. Further, this condition appears to allow a closerapproach to equilibrium insofar as the recrystallization tendencies areconcerned, allowing the anode surface, although polycrystalline, toattain a nature approaching that of a nearly perfect single crystal.

In the Work that was carried out on this invention, electricalproperties of the anodized specimens were tested in an electrolyteconsisting of 10% H3PO. Leakage current was measured at 25 C. after 5minutes of electrification at of forming potential. Capacitance anddissipation factors were measured with a 120 c.p.s. 1 volt A.C. signalunder a 10 volt DC. bias at 25 C.

For a clearer understanding of the invention, the following specificexamples are given. These examples are intended to be merelyillustrative of the invention and not in limitation thereof. Unlessotherwise specified, all parts are by weight.

EXAMPLES 1 THROUGH 13 Columbium-titanium alloys consisting of, byweight, columbium and 10% titanium; 70% columbium and 30% titanium; 60%columbium and 40% titanium; 50% columbium and 50% titanium; 40%columbium and 60% titanium; and 30% columbium and 70% titanium wereprepared by melting and remelting together proportionate amounts ofthese metals. The resulting homogeneous alloy buttons were rolled to0.007" foil by repetitive rolling operations but without intermediateannealing and from these foils test specimens /2 x 1" were cut. Thesepieces were precleaned by washing in trichloroethylene, acetone, anddistilled water, and air dried. The specimens were heated in vacuum attemperatures of 1700 C., 1800 C., 1900 C., 2000 C., and 2100 C., for onehour. Following these heat treatments, the specimens were anodized,using an electrolyte comprising 0.01% H PO in water. The current densityused to form the oxide film was 200 milliamps/in and the oxide film wasformed to 200 v.

The specimens were removed from the electrolyte, washed in distilledwater at 90 C., for one hour, and air dried. They were then tested forelectrical properties, and these are given in Table I.

For comparison of electrical properties of the heattreated specimenswith those of the control samples, specimens cut from rolled foil weretreated and tested in exactly the same way as the alloy samplesdescribed above except that the step of heat treating in vacuum wasomitted. The capacitance of these unheated samples and the leakagecurrents are also given in Table I.

T able I.-Electrical properties of anodized specimens derived fromNb-Base alloys [ELECTRICAL PROPERTIES AFTER ANODIZATION] Alloy comp.Heat before heattreatment ing (wt. prior to Percent Capaci- LeakageDissipation Example percent) anodizatiou wt. loss on tance current.factor,

(1 hr.-vacheating id/in. ,uiL/yfd. V. percent uum), 0. Nb Ti.

Control. 100 0. 98 0.0060 1.00 Control 70 30 0.52 0.144 Control 50 500.57 5.17

*Not determined.

EXAMPLE 14 Table II A foil was prepared by meltmg together 40 parts byAlloy composition Heat treat I Weight of titanium metal powder and 60parts by welght 30 t n? hetat 111 11121 priior to Capacit Leakagte reamen .ano iza'ion ance, 1 curren of columbium metal powder, and rollingthe resultlng (I mfvacuumx pfd'vllinfi MEL/Id. lngotto a thickness of0.007 I as in Examples 1-13. C. Photomicrographs of the foil at 500showed a smooth Nb Ta V surface. A portion of this foil was then treatedin 75 2f 1 700 1 30 0 0202 o 0 vacuum at 1700 C. for a period of onehour, and photov 75 25 1,900 216 Q0306 micrographs were made whichshowed substantial porosity l 75 25 2,300 3. 32 0. 2022 in the surface.Three other foil specimens were also cut 55 5 gig ,15 from thesame'rolled piece, and one heated in vacuum for 75 25 2,100 3. 50 0.00751 hour at 1800" 0, one for 1 hour at 1900 C., and one for 1 hour at 2100C. An increase in surface area, as 40 heat treatment beforeanodiwtionindicated by the porosity of the surface, in photomicrographsat 500 showed that the surface area increased with increasingtemperature of heat treatment. The foil heated at 2100 C. showed thegreatest surface area. X'-ray fluorescence intensity measurements on thesurface of the foil after heating for one hour in vacuum at 2l00 C.established that there was less than 0.1% titanium on the surface. TheX-rays in this test penetrate the metal surface to a depth of 0.003".

In a similar manner, an alloy foil of 50% by weight columbium and 50% byweight titanium was'prepared by melting and rolling a foil of 0.007"thickness from the ingot. Portions of this foil were heated under vacuumfor one hour at 17 00 C., 1800'C., 1900 C., and 2000 C. Portions ofthese heated specimens were examined under the microscope andphotomicrographs were made. These showed that surface-roughening andporosity resulted in these specimens due to the evaporation of the lowermelting metal, and this roughening and porosity increased with heatingat higher temperatures.

EXAMPLE 15 Separate specimens of an alloy consisting essentially of 75%tantalum and 25% titanium were heated at 1700 C., 1900 C., and 2100 C.for one hour in a vacuum and in each instance the alloy was anodized andthen tested in the same manner as previously described. Capacitorcomponents were also prepared in the same manner from alloys consistingessentially of 75 columbium and 25% vanadium; and from 75 tantalum and25 vanadium. The results of heating these alloys under vacuum, anodizingthe resulting porous foil, and testing in the same manner as previouslydescribed, are summarized in Table II.

From the above, it is seen that useful electrolytic capacitor componentscan be prepared from either tantalum or niobium alloyed with eithervanadium or titanium. It

is also contemplated to use combinations of these materials.

Since one purpose of this invention is to improve the capacitancecapability of the porous metal body, the most direct Way tocharactertize the products .of this invention is in terms ofthe'superior capacitance of capacitors made therefrom. From the datapresented in Table I, it will be seen that among the prior art controlmaterials tested, unalloyed columbium showed the highest capacitance perunit area; that is to say, apparent, projected or geometric area. It canalso be seen from Table I that the present invention produces materialshaving at least 4% more capacitance per unit area than that fromunalloyed columbium (see Example 1). Moreover, in the case of Example 7,there is over a 15-fold increase in capacitance as compared withunalloyed columbium.

The increased capacitance obtainable according to this invention is aresult of the porous nature of the surface of the metal. This porosityis caused by the escape of the metal body during heating, leaving anuneven surface of increased surface area. This increased surface areacan be measured by the conventional Brunauer-Emmet- Teller GasAdsorption Method, using krypton as the adsorbed gas. The ratio of theB.E.T. measured area to the geometric surface area, as determined by thelength and width dimensions of the metal piece, gives a numerical valuewhich is referred to as the roughness factor. Products prepared by thisinvention have roughness factors of from 3 to 15 or higher, and thesefactors do not change substantiallythat is, by more than about 10%, whenthe products are anodized to 200 volts. In contrast, conventionalnon-etched metal foils have a roughness factor which does not exceed1.6.

Among the products treated according to this invention were 3 foilshaving a thickness of 2, 4, and 7 mils, respectively, made from a 50%Nb-50% Ti alloy. The two-mil foil was held under vacuum at 2000 C. for15 minutes; the 4-mil foil was held under vacuum at 1900 C. for 30minutes; and the 7-mil foil was held under vacuum at 2000 C. for 1 hour.The roughness factors were found 1 to be 3.74, 6.15 and 12.2,respectively, by B.E.T. adsorption measurements. The capacitance valuesafter anodization were found to be 4.71, 10.43, and 15.4 times the valueof that of a pure columbium foil anodized under the same conditions.

It will be seen from the results given in Table I above that the anodeswhich have been prepared according to the process of this inventionexhibit exceptionally low dissipation factors considering the largesurface enhancement factors. This improvement is attributed to thenature or character of the pores developed in the surface of theelectrode metal by the methods of this invention. The surfaces containpores of an open structure with smooth walls and rounded closed ends, sothat permeation of electrolyte when anodizing the electrode isfacilitated with formation of anodic oxide films on all surfaces.Similarly, in finished capacitor devices, the counterelectrode, whetherliquid, gel, or semiconducting solid, has ready access to the wholehighly developed surface of the electrodes of this invention. Whenenhancement of the surfaces of capacitor electrode materials isattempted by etching with acids, a procedure by which the metals areattacked preferentially at metal grain boundaries, removal of metal fromthese boundaries increases the effective surface but not to the extentdescribed in this invention; furthermore, the pores so produced are of aV-shaped or crevasse-like character which do not so readily permit fullpenetration by forming electrolytes or counterelectrolytes, and which asa consequence of their shape, lead to higher dissipation factors on thetinished device. A further disadvantage of enhancedsurfaee metal bodiesproduced by etching is that the crevasse-like pores are soon filled upwith oxide in the course of anodification and therefore the full benefitof surface area enhancement achieved by this means is lost upon formingto higher voltages. By contrast, the effective area of surface-enhancedanodes made according to this invention is independent of formingvoltage up to at least 200 volts.

The novel alloys can be used as electrodes in whatever shape desired.Electrodes are commonly prepared from foil, but in certain instances itmay be convenient to use the alloy in the form of wire, or in anygeometric form appropriate to the design of the capacitor.

In the anodization of the specimens described above, the use ofphosphoric acid has been found to be a satisfactory forming and testingelectrolyte. However, the invention is not limited to the use of this orany other specific forming or testing electrolyte as the improvedproperties in the capacitor component parts are not in general dependentupon the use of any specific electrolyte.

I claim:

1. A porous metal product having a roughness factor which is at least 3and remains substantially unchanged upon anodization of the product to200 v., said product comprising a porous metal body having pores thereinoriginating in cavities near the body surface, the cavities openingthrough channels to the outer periphery of the body, the surface of saidbody, including the surface of the pores therein, consisting essentiallyof a metal from the group consisting of columbium, tantalum, andcombination thereof, the purity of said surface metal being such thatwhen the body is anodized to 200 volts at 25 C. by electrolyzing it in a.01% by Weight solution of orthophosphoric acid in water, theconcentration of contaminants in the anodic oxide film so formed on itssurface is below about 1% by weight, constituents other than columbium,tantalum and oxygen being considered contaminants, and there being inregions of said porous body away from its surface, in addition to themetal of the group consisting of columbium, tantalum and corn binationsthereof, a metal of the group consisting of titanium, vanadium, andcombinations thereof.

2. A porous metal product of claim 1 wherein the metal is columbium.

3. A porous metal product of claim 1 wherein the metal is tantalum.

4. An electrolytic capacitor comprising as an anode therein an anodizedporous metal product of claim 1.

References Cited by the Examiner UNITED STATES PATENTS 2,447,980 8/48Hensel 148-13 X 2,822,268 2/58 Hix 174 2,940,845 6/60 Jafee et al. 751742,964,399 12/60 Lyons 75174 3,098,955 7/63 Davis et a1. 317242 FOREIGNPATENTS 1,097,690 1/61 Germany.

OTHER REFERENCES Miller, Tantalum and Niobium, 1959, pages 4559,published by Academic Press Inc.

DAVID L. RECK, Primary Examiner.

1. A POROUS METAL PRODUCT HAVING A ROUGHNESS FACTOR WHICH IS AT LEAST 3AND REMAINS SUBSTANTIALLY UNCHANGED UPON ANODIZATION OF THE PRODUCT TO200 V., SAID PRODUCT COMPRISING A POROUS METAL BODY HAVING PORES THEREINORIGINATING IN CAVITIES NEAR THE BODY SURFACE, THE CAVITIES OPENINGTHROUGH CHANNELS TO THE OTHER PERIPHERY OF THE BODY, THE SURFACE OF SAIDBODY, INCLUDING THE SURFACE OF THE PORES THEREIN, CONSISTING ESSENTIALLYOF A METAL FROM THE GROUP CONSISTING OF COLUMBIUM, TANTALUM, ANDCOMBINATION THEREOF, THE PURITY OF SAID SURFACE METAL BEING SUCH THATWHEN THE BODY IS ANODIZED TO 200 VOLTS AT 25*C. BY ELECTROLYZING IT IN A.01% BY WEIGHT SOLUTION OF ORTHOPHOSPHORIC ACID IN WATER, THECONCENTRATION OF CONTAMINANTS IN THE ANODIC OXIDE FILM SO FORMED ON ITSSURFACE IS BELOW ABOUT 1% BY WEIGHT, CONSTITUENTS OTHER THAN COLUMBIUM,TANTALUM AND OXYGEN BEING CONSIDERED CONTAMINANTS, AND THERE BEING INREGIONS OF SAID POROUS BODY AWAY FROM ITS SURFACE, IN ADDITION TO THEMETAL OF THE GROUP CONSISTING OF COLUMBIUM, TANTALUM AND COMBINATIONSTHEREOF, A METAL OF THE GROUP CONSISTING OF TITANIUM, VANADIUM, ANDCOMBINATIONS THEREOF.